1. Field of the Invention
The present invention relates generally to implantable medical devices and more specifically relates to body implantable electrodes.
2. Description of the Prior Art
By far the majority of body implantable leads in use, particularly in the pacing field, utilize metallic electrodes. A favored material for such metallic electrodes is platinum or a platinum alloy. Other materials which have been used include stainless steel, titanium and so forth. Care must be exercised in chosing these materials to assure that they are biocompatible and capable of long-term chronic implantation. For that reason, materials in the platinum group have been preferred notwithstanding their relatively high cost.
The use of other than solid metallic electrodes has been relatively rare. Lagergren in U.S. Pat. No. 3,911,928 teaches an electrode surface alternately covered with highly conducting metallic materials and insulating materials. These materials are arranged in the fashion of a mesh. It is expressed by Lagergren that such an arrangement is desirable as it provides a large contact surface for sensing purposes and yet presents a relatively small contact surface for stimulating purposes thereby producing sufficiently large charged densities.
Other persons have taught the use of materials which are not highly conducting metals. By far the most often preferred non-metallic material is carbon. Thoren in U.S. Pat. No. 4,149,542 teaches an endocardial electrode having a carbon tip. The purpose of using the carbon tip is stated as providing a heart tissue compatible conductive material. The concern herein is the growth of fibrotic tissue and therefore, carbon is chosen because it is thought to be highly biocompatible. However, Thoren teaches no selection of material based upon its conductivity.
Richter, in Offenlegungschrift No. 28 42 318 teaches an implantable carbon electrode. Again, the concern is that of body compatibility. Richter teaches that metal stimulation electrodes are less desirable because they cause slow degeneration of the tissue adjoining the electrodes. Therefore, it is felt that a carbon tip electrode, because it is more biocompatible, will provide a lower overall chronic energy threshhold.